Magnesium and its alloys are useful industrial materials principally due to the light weight and high strength to weight ratios which characterize them. Nevertheless, these materials possess disadvantages which inhibit their use in many applications. Thus, the alloys are comparatively soft and are subject to galling and seizing when engaged in rubbing friction under load. The modulus of the alloys also is lower than that which would be desirable in certain applications. Property improvements have been achieved through the use of alloying additions but even further improvements would be of benefit.
Pressures to provide even greater property improvements together with the provision of property combinations heretofore unobtainable have lead to consideration of magnesium and its alloys as a constituent of a composite system. As an example, greater strengths have been obtained in aluminum alloy materials by using alumina fibers bonded to an aluminum alloy matrix as taught in U.S. Pat. No. 4,012,204.
Methods commonly used to prepare metal-matrix composite materials may be classified into three categories; namely,
(1) Solid-state or semi-solid-state consolidation PA1 (2) Pressure infiltration or squeeze casting PA1 (3) Casting; a process in which reinforcing materials, normally having little or no solubility in the matrix material, are mixed with the matrix metal or alloy at a temperature above the liquidus temperature of the matrix material. The molten mixture containing reinforcing material in suspension is then solidified. It is essential that the reinforcing material be wetted by the melt, as otherwise it will be rejected and no reinforcement will result. This has been recognized, for example, in U.S. Pat. No. 3,885,959 which teaches coating the surface of the reinforcing particles with nickel to promote wetting.
Technical development of the casting method is less advanced than the methods of Categories 1 and 2. The technique offers advantages in applications for producing relatively large size ingots at reasonable cost.